1. Field of the Invention
The present invention relates to a battery pack including a plurality of battery cells stored in a case, and more specifically, to a battery pack capable of restraining a rise and variation in temperature of battery cells, thereby ensuring steady charging and reliable cell performance.
2. Description of the Prior Art
Nickel-metal hydride secondary battery cells are used as power sources for modern electro-vehicles, power-assisted bicycles, power tools, etc. Conventionally, the cells of this type are provided in the form of a battery pack in which a plurality of battery cells that are connected in series or parallel with one another are stored in a case that is formed of polycarbonate or ABS resin, for example.
In order to enjoy a compact general configuration, the conventional battery pack is designed so that the battery cells are stored most closely in contact with one another in a case, whereby the capacity efficiency is improved. In the case of columnar cells, twelve battery cells 1 are arranged alternately in three rows with their respective peripheral surfaces in contact with one another so as to minimize the storage capacity for them, and are stored in a rectangular case 2, as shown in FIG. 19, for example.
In the battery pack of this conventional construction, variation in temperature is easily caused between the battery cells 1 that are arranged along the wall surface of the case 2 and the ones that are located in the central portion. Although heat generated in the cells 1 that are arranged along the case wall surface can be readily released through the wall surface, heat produced in the central cells is liable to be accumulated therein. The variation in the cell temperature causes dispersion in the properties (capacities, in particular) of the individual battery cells 1. If the battery pack is used with the battery cells 1 subject to the dispersion in properties, the respective capacities of the cells 1 are gradually exhausted, the smallest one first. The exhausted battery cells 1 may undergo some troubles, such as pole reversal, increase in internal resistance attributable to dissipation of the electrolyte solution, etc. The dissipation of the electrolyte solution is attributed solely to gas discharge from a relief valve that occurs as the internal pressure of the battery cells 1 increases. In consequence, troubles may be caused including lowering of the cell properties, failure of recharge, etc. Thus, the battery pack is lowered in performance, and its life is shortened.
In the case where the battery cells 1 are nickel-metal hydride secondary battery cells, they generate great heat as they are charged. The battery cells 1 generate particularly intensive heat when they are charged with high current in a short period of time. Further, the charging efficiency and cell capacity of the nickel-metal hydride secondary battery cells lower as the cell temperature increases. As the cell temperature rises, moreover, a hydrogen-storing alloy that forms a negative electrode easily corrodes in the electrolyte solution, so that its hydrogen occluding/releasing capacity lowers. It is essential, therefore, to prevent the rise in temperature of the battery cells (nickel-metal hydride secondary battery cells) 1 in the case 2, especially when the cells are charged.
Since the power tool are handled roughly, in general, the battery pack that is used as its power source is expected to be structurally resistant to external impact such as drop impact.
An object of the present invention is to provide a battery pack capable of preventing or restraining a rise and variation in temperature of battery cells stored in a case, thus fulfilling its cell performance.
Another object of the invention is to provide a battery pack capable of being safely charged in a manner such that a rise in temperature of cells is effectively restrained during the charge.
Still another object of the invention is to provide a power tool capable of effectively cooling a battery pack.
According to the present invention, there is provided a battery pack having a plurality of battery cells stored side by side in a case, the case including a bottomed trough and a lid, the trough having a space as a cell storage portion defined between inner and outer wall portions paired to form a loop and a central space as a hollow surrounded by the inner wall portion and penetrating the trough from top to bottom, the lid closing a top opening of the cell storage portion. Heat generated in the battery cells is released through the hollow from the inner wall portion the trough as well as from the outer wall portion. Thus, the rise and variation in temperature of the battery cells can be restrained.
Preferably, the trough is designed so that the pillar-shaped battery cells stored in the cell storage portion are arranged side by side in a loop with their respective outer peripheral surfaces in contact with one another, so that heat in the battery cells can be released through the outer and inner wall portions. Heat generation during charge can be effectively restrained even in the case where a nickel-metal hydride secondary battery cell is used as each battery cell.
The battery pack according to the invention may further comprise a terminal block overlying the battery cells stored in the cell storage portion of the trough and having electrode leads of the group of battery cells led out therefrom, and may be designed so that the lid has apertures in those regions thereof which face the electrode leads, through which the electrode leads are exposed for external connection.
Preferably, electrode leads of the terminal block are arranged in a region opposite the hollow. The apertures through which the electrode leads are exposed for external connection are arranged along the inner peripheral portion of the lid opposite the hollow.
In particular, the terminal block is provided with a pair of electrode lead terminals, positive and negative, connected individually to the electrode leads of the cell group and one or a plurality of auxiliary terminals for the cell group.
The auxiliary terminals are used when the cell group is charged. More specifically, they include a terminal for charging connected in series with the cell group through temperature protection elements for cutting off a charging path for the cell group in response to the temperature of the cells, a terminal for temperature detection connected to a temperature transducer for detecting the cell temperature, and a terminal for cell type identification connected to a resistor having a resistance value corresponding to the specifications of the cells.
The temperature protection elements are provided individually in a plurality of spots in the direction of arrangement of the battery cells stored in a loop in the cell storage portion of the trough. They are interposed in series between the cell group and the charging terminal.
The inner wall portion of said trough is inclined outward from the base portion thereof to the top opening, whereby the sectional area of the top opening of the hollow surrounded by the inner wall portion is reduced gradually. By reducing the sectional area in this manner, a current of air circulating in the hollow can be speeded up on the upper end side, whereby the cooling efficiency can be enhanced. In this case, the inner wall portion should be inclined at an angle of 0.5xc2x0 to 5xc2x0 to the vertical direction. Further, the inner and outer wall portions of the trough should only be as high as the cylindrical battery cells.
The battery pack constructed in this manner may be attached integrally to, e.g., the lower part of a handgrip portion of a power tool to be used as a power source thereof. It is to be desired that the cooling efficiency for the cell group should be enhanced by circulating air in the hollow of the battery pack by utilizing the rotation of a motor.
According to the present invention, there may be provided a battery pack that can store a plurality of battery cells with structural stability and restrain a rise and variation in temperature of the battery cells, thus fulfilling its cell performance. Further, the charging state of the battery cells, especially the cell temperature, can be observed as the charge of the battery cells is safely controlled.
Since the battery cells can be kept stable, moreover, the resulting battery pack has a structure that is highly resistant to external impact and the like, and the power tool is easy to handle.